This invention relates to tomosynthesis and, more particularly, to a method and apparatus for performing tomosynthesis by acquiring 2-D x-ray projection images of an object, transforming the data into a form as if it had been acquired by a computed tomography (CT) system, and then reconstructing a 3-D representation of the object by utilizing a CT image reconstruction algorithm. By utilizing a CT reconstruction algorithm to reconstruct an image from image data acquired by a tomosynthesis system, the quality of the reconstructed images is greatly improved.
Digital tomosynthesis enables a three dimensional (3-D) image of an object to be constructed from a finite set of two-dimensional (2-D) projection radiograph images. The system comprises an x-ray source and a 2-D x-ray detector, which is a digital detector. In typical digital tomosynthesis systems, during data acquisition, the x-ray source is rotated by a gantry on an arc through a limited angular range about a pivot point and a set of projection radiographs of the object are acquired by the detector at discrete locations of the x-ray source. The detector is maintained at a stationary position as the radiographs are acquired.
Once the projection radiographs have been obtained, they are then spatially translated with respect to each other and superimposed in such a manner that the images of structures in the tomosynthesis plane overlap exactly. The images of structures outside the tomosynthesis plane do not overlap exactly, resulting in a depth dependent blurring of these structures. By varying the amount of relative translation of the projection radiographs, the location of the tomosynthesis plane can be varied within the object. Each time the tomosynthesis plane is varied, the image data corresponding to the overlapping structures is superimposed and a 2-D image of the structure in the tomosynthesis plane is obtained. Once a complete set of 2-D images of the object has been obtained, a 3-D image of the object is generated from the set of 2-D images.
Computed tomography (CT) is a technique that generally involves subjecting a patient to x-rays, acquiring digital x-ray data of a portion of the patient's body, and processing and back-projecting the digital x-ray data to construct a 3-D image of the object. The image may then be displayed on a display monitor of the CT system. CT systems typically comprise a gantry, a table, an x-ray tube, an x-ray detector array, a computer and a display monitor. The computer sends commands to controllers of the gantry to cause the controllers to rotate the x-ray tube and/or the detector array at a particular rotational speed to produce relative rotation between the x-ray tube and the detector array 360.degree. about the patient. The detector array is usually comprised of either a curved array (third generation CT system) of detector elements or a ring (fourth generation CT system) of detector elements. In the case where a ring of detector elements is used, only the x-ray tube is rotated.
As stated above, in digital tomosynthesis, the acquired projection radiographs are spatially translated and superimposed such that the structures in the tomosynthesis plane overlap. The images of structures outside of the tomosynthesis plane do not coincide precisely when they are superimposed, which results in a depth dependent blurring of these structures. These out-of-plane structures are superimposed into the reconstructed plane, which degrades the overall reconstructed image quality and results in relatively low depth resolution. In contrast, the filtered back projection reconstruction algorithms utilized by CT systems to reconstruct a 3-D image from acquired 2-D image slices generally produces reconstructed images of much higher quality and better depth resolution than the reconstructed images produced by using digital tomosynthesis systems.
It would be desirable to provide a digital tomosynthesis system that could perform reconstruction using a filtered back projection algorithm such as that used in CT systems. However, due to the differences between digital tomosynthesis and CT geometries, image reconstruction algorithms that are normally utilized by CT systems generally are not suitable for tomosynthesis reconstruction. Accordingly, a need exists for a digital tomosynthesis system that is capable of utilizing image reconstruction algorithms of the type typically used with CT systems to perform reconstruction.